The 1946 Aleutian Islands earthquake occurred near the
Aleutian Islands
The Aleutian Islands (; ; ale, Unangam Tanangin,”Land of the Aleuts", possibly from Chukchi ''aliat'', "island"), also called the Aleut Islands or Aleutic Islands and known before 1867 as the Catherine Archipelago, are a chain of 14 large v ...
,
Alaska
Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U ...
on April 1, 1946. The shock had a
moment magnitude () of 8.6, a
tsunami magnitude of 9.3, and a
surface-wave magnitude () of only 7.4, and a maximum
Mercalli intensity of VI (''Strong'').
It resulted in 165–173 casualties and over $26 million in damage. The seafloor along the fault was elevated, triggering a Pacific-wide
tsunami with multiple destructive waves at heights ranging from . The tsunami obliterated the
Scotch Cap Lighthouse on
Unimak Island, Alaska among others, and killed all five lighthouse keepers. Despite the destruction to the Aleutian Island Unimak, the tsunami had almost an imperceptible effect on the Alaskan mainland.
Tectonic setting
The Aleutian Islands are a group of 14 large and 55 smaller volcanic islands situated between mainland Alaska and
Kamchatka.
They are situated along the
Aleutian Trench as they were formed by the active subduction of the oceanic
Pacific plate underneath the continental
crust of the
North American plate
The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Paci ...
.
The western portion of the trench becomes more and more oblique until near the
Commander Islands where faulting becomes nearly exclusively
strike-slip and causes earthquakes such as the
2017 Komandorski Islands earthquake.
The trench ends where the
Kuril-Kamchatka Trench begins in the west.
In the east, the trench extends for a couple thousand miles before reaching the transition zone near
Yakataga responsible for creating earthquakes such as the
1899 Yakutat Bay earthquakes.
The collision totally ends slightly further east, where the
Queen Charlotte Fault becomes the dominant fault of the tectonic regime.
This specific area of the Aleutian Islands is thought to have ruptured in the
1585 Aleutian Islands earthquake.
Earthquake
The earthquake struck at 3:29 am local time on April 1, 1946, at a shallow depth of . The earthquake was very large, but the maximum felt intensity was only a VI. The earthquake was originally thought to be a rather smaller earthquake of 7.4, however further research uncovered that this was truly a much larger event, but with a hidden seismic signature.
A size of 8.6 fits this event best, with an estimated slip between and .
This event shows the classic signs of a
tsunami earthquake as the surface-wave magnitude was very low compared to the moment and tsunami magnitudes, and the tsunami height was far larger than expected for the surface wave magnitude.
The very high tsunami magnitude value is due to the sheer strength of the waves. It was calculated by using tsunami run up from
Honolulu
Honolulu (; ) is the capital and largest city of the U.S. state of Hawaii, which is in the Pacific Ocean. It is an unincorporated county seat of the consolidated City and County of Honolulu, situated along the southeast coast of the isla ...
, Hilo, and the average of stations in California. Even excluding the values given by Hilo (the highest ), the tsunami magnitude is at least 9.1.
The earthquake was originally thought to be a strike-slip earthquake, and later a strike slip earthquake with a normal faulting component. These erroneous
focal mechanisms were due to a lack of stations recording the earthquake. However, modeling of the
S wave
__NOTOC__
In seismology and other areas involving elastic waves, S waves, secondary waves, or shear waves (sometimes called elastic S waves) are a type of Linear elasticity#Elastic wave, elastic wave and are one of the two main types of elastic ...
, rather than the
P wave helped to uncover both the true magnitude of the earthquake, as well as the widely accepted thrusting mechanism. This thrust mechanism is consistent with a subduction earthquake helping to relieve stress on the Aleutian Trench.
Tsunami
At Unimak Island, tsunami run-up reached . The tsunami earthquake aspect of this event helps to explain a portion of this massive height, however the rest must be attributed to a local submarine landslide.
Waves reportedly traveled across the ocean at and measured high, crest to trough. The wave reached
Kauai, Hawaii 4.5 hours after the quake, and
Hilo, Hawaii 4.9 hours later. In Hilo, 173 died, 163 were injured, 488 buildings were demolished and 936 more were damaged. Witnesses told of waves inundating streets, homes, and storefronts. Many victims were swept out to sea by receding water. The tsunami caused much damage in
Maui as well. Waves there demolished 77 homes and many other buildings. The residents of these islands were caught off-guard by the onset of the tsunami due to the inability to transmit warnings from the destroyed posts at Scotch Cap, and the tsunami is known as the April Fools' Day Tsunami in Hawaii because it happened on April 1. The effects of the tsunami also reached
Washington
Washington commonly refers to:
* Washington (state), United States
* Washington, D.C., the capital of the United States
** A metonym for the federal government of the United States
** Washington metropolitan area, the metropolitan area centered o ...
,
Oregon
Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...
, and
California
California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
.
The tsunami was unusually powerful for the size of the earthquake. The event was classified as a tsunami earthquake due to the discrepancy between the size of the tsunami and the relatively low surface-wave magnitude.
The large-scale destruction prompted the creation of the Seismic Sea Wave Warning System, which became the
Pacific Tsunami Warning Center in 1949.
See also
*
List of earthquakes in 1946
This is a list of earthquakes in 1946. Only magnitude 6.0 or greater earthquakes appear on the list. Lower magnitude events are included if they have caused death, injury or damage. Events which occurred in remote areas will be excluded from th ...
*
List of earthquakes in Alaska
This is an incomplete list of earthquakes in Alaska.
See also
* Geology of Alaska
References
{{Authority control
Earthquakes
Alaska
Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alas ...
*
List of earthquakes in the United States
References
External links
*
Tsunami Animation: Unimak Island, Aleutian Islands, 1946–
Pacific Tsunami Warning Center
1946 Aleutians Tsunami– Western States Seismic Policy Council
– ''
LiveScience''
*
Survivor audio interviewtranscript
{{DEFAULTSORT:Aleutian Islands earthquake, 1946
1946 earthquakes
1946 in Alaska
1946 in Hawaii
1946 tsunamis
Earthquake
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...
Megathrust earthquakes in Alaska
1946 Aleutian
April 1946 events in the United States
Tsunami earthquakes
History of Hawaii